Probably because IRV is easier to actually do and doesn't really have any "free variables" in how you should behave. For example if I strongly like candidate A and I like candidate B a bit less then I should obviously rank them first and second. What about scoring? Should I give them 10, 9? But will that penalise B when A is eliminated? Similarly should I give them Good, Good, or Good, Approve?

You kind of have to keep the whole voting system model in your head to decide.

You might not think it's very difficult but even ranking candidates is too complicated for most people. Anything more complicated than that has no chance in the real world.